1. Field of the Invention
The present invention relates to medical methods and systems. More particularly, the invention relates to a method and system for physiological gating of radiation therapy.
2. Background
Radiation therapy involves medical procedures that selectively expose certain areas of a human body, such as cancerous tumors, to high doses of radiation. The intent of the radiation therapy is to irradiate the targeted biological tissue such that the harmful tissue is destroyed. In certain types of radiotherapy, the irradiation volume can be restricted to the size and shape of the tumor or targeted tissue region to avoid inflicting unnecessary radiation damage to healthy tissue. For example, conformal therapy is a radiotherapy technique that is often employed to optimize dose distribution by conforming the treatment volume more closely to the targeted tumor.
Normal physiological movement represents a limitation in the clinical planning and delivery of conventional radiotherapy and conformal therapy. Normal physiological movement, such as respiration or heart movement, can cause a positional movement of the tumor or tissue region undergoing irradiation. If the radiation beam has been shaped to conform the treatment volume to the exact dimensions of a tumor, then movement of that tumor during treatment could result in the radiation beam not being sufficiently sized or shaped to fully cover the targeted tumoral tissue.
To address this problem, the size and/or shape of the radiation beam can be expanded by a xe2x80x9cmovement marginxe2x80x9d (i.e., the predicted movement distance in any direction of the targeted tumor) to maintain full irradiation of the targeted tissue. The drawback to this approach is that this increased irradiation volume results in radiation being applied to otherwise healthy tissue that is located within the area of the expanded volume. In other words, motion during treatment necessitates the application of a radiation field of an expanded size that could negatively affect an unacceptably large volume of normal tissue surrounding the targeted treatment volume.
Another approach to this problem involves physiological gating of the radiation beam during treatment, with the gating signal synchronized to the movement of the patient""s body. In this approach, instruments are utilized to measure the physiological state of the patient with reference to the particular physiological movement being examined. For example, respiration has been shown to cause movements in the position of a lung tumor in a patient""s body. If radiotherapy is being applied to the lung tumor, then a temperature sensor, strain gauge or preumotactrograph can be attached to the patient to measure the patient""s respiration cycle. The radiation beam can be gated based upon certain threshold points within the measured respiratory cycle, such that the radiation beam is disengaged during periods in the respiration cycle that correspond to excessive movement of the lung tumor.
Known methods for performing physiological gating typically require specialized instruments to be placed in contact with or invasively mounted on the patient. For instance, the known approaches to physiological gating synchronized with the respiratory cycle require a patient-contact instrument such as a strain gauge or spirometer to be attached to the patient""s body. A known approach to gating synchronized to the cardiac cycle requires an electrocardiograph to be connected to the patient""s body. Requiring an instrument to be placed in contact with, or invasively mounted in, the patient""s body could cause problems under certain circumstances. For example, a spirometer is a pneumotachograph device that is mounted on a patient to measure the volume of air passing through the patient""s airway during respiration. The discomfort associated with using a spirometer can limit the usefulness of that instrument in measuring a patient""s respiration cycle, particularly if the gating procedure requires use of that instrument for an extended period of time. Moreover, many of these instruments have cumbersome wires or connections that limit the usability of these instruments within certain confined areas or with certain patient body configurations. Another drawback is that a specialized instrument is required for each body part that is being measured for movement. Not only does this require the use of a plurality of instruments for the multiplicity of body parts that may have to be measured for movement, but in some cases, the particular body part undergoing examination may not have an associated specialized instrument to detect its movement.
Therefore, there is a need for a system and method to address these and other problems of the related art. There is a need for a method and system of physiological gating which does not require instruments or probes to be mounted, either externally or invasively, on or in the patient""s body. Moreover, there is a need for a method and system that can accurately and consistently allow planning for physiological gating of radiation treatments.
The present invention provides an improved method and system for physiological gating for radiation therapy. According to an aspect, the invention comprises the use of an optical or video imaging system to generate image data to measure regular physiological movement of a patient""s body. An optical or video imaging system provides a non-invasive method for measuring motion on a patient""s body. The image data can be used to quantify voluntary or involuntary motion of the patient that may affect the delivery of radiation to a target valve. A gating signal can be generated to suspend delivery of radiation upon certain threshold event detected in the motion cycle.